1. Field of the Invention
The invention relates to highly crystalline syndiotactic polypropylene and, particularly, to syndiotactic polypropylene having a syndiotactic index of greater than 75%.
2. Description of the Prior Art
As known in the art, syndiotactic polymers have a unique stereochemical structure in which monomeric units having enantiomorphic configuration of the asymmetrical carbon atoms follow each other alternately and regularly in the macromolecular main chain. Syndiotactic polypropylene was first disclosed by Natta et al, in U.S. Pat. No. 3,258,455. The Natta group obtained syndiotactic polypropylene by using a catalyst prepared from titanium trichloride and diethyl aluminum monochloride. A later patent to Natta et al, U.S. Pat. No. 3,305,538, discloses the use of vanadium triacetylacetonate or halogenated vanadium compounds in combination with organic aluminum compounds for producing syndiotactic polypropylene. U.S. Pat. No. 3,364,190 to Emrick discloses a catalyst system composed of finely divided titanium or vanadium trichloride, aluminum chloride, a trialkyl aluminum and a phosphorus-containing Lewis base as producing syndiotactic polypropylene.
As disclosed in these patent references and as known in the art, the structure and properties of syndiotactic polypropylene differ significantly from those of isotactic polypropylene. The isotactic structure is typically described as having long sequences of monomer units with the same relative configuration of the tertiary carbon atoms. Using the Fischer projection formula, the stereochemical sequence of isotactic polypropylene is described as follows: ##STR1## The methyl groups attached to the tertiary carbon atoms of successive monomeric units on the same side of a hypothetical plane through the main chain of the polymer, e.g., the methyl groups are all above or below the plane.
Another way of describing the structure is through the use of NMR. Bovey's NMR nomenclature for an isotactic pentad is . . . mmmm . . . with each "m" representing a "meso" dyad or successive methyl groups on the same side in the plane. As known in the art, any deviation or inversion in the structure of the chain lowers the degree of isotacticity and crystallinity of the polymer.
In contrast to the isotactic structure, syndiotactic polymers are those in which long sequences of monomer units have an alternating relative configuration of the tertiary carbon atoms. Using the Fischer projection formula, the structure of a syndiotactic polymer is designated as: ##STR2## The methyl groups attached to the tertiary carbon atoms of successive monomeric units in the chain lie on alternate sides of the plane of the polymer.
In NMR nomenclature, this pentad is described as . . . rrrr . . . in which each "r" represents a "racemic" dyad, i.e., successive methyl groups on alternate side of the plane. The percentage of r dyads in the chain determines the degree of syndiotacticity of the polymer. Syndiotactic polymers are crystalline and like the isotactic polymers are insoluble in xylene. This crystallinity distinguishes both syndiotactic and isotactic polymers from atactic polymer that is soluble in xylene. Atactic polymer exhibits no regular order of repeating unit configurations in the polymer chain and forms essentially a waxy product.
While it is possible for a catalyst to produce all three types of polymer, it is desirable to produce predominantly isotactic or syndiotactic polymer with very little atactic formed. Catalysts that produce isotactic polyolefins are disclosed in European Patent Application No. 87870132.5 (Publication No. 0 284 308) published Oct. 5, 1988 and U.S. Pat. Nos. 4,794,096 issued Dec. 27, 1988 and 4,975,405 issued Dec. 4, 1990. These applications disclosed chiral, stereorigid metallocene catalysts that polymerize olefins to form isotactic polymers and are especially useful in the polymerization of a highly isotactic polypropylene. The present invention, however, utilize a different class of metallocene catalyst that are useful in the polymerization of syndiotactic polyolefins, and more particularly syndiotactic polypropylene.
The present invention provides highly crystalline syndiotactic polypropylene. It was discovered that the catalyst structure affected the formation of a syndiotactic polymer as opposed to an isotactic polymer. One theory, without limiting the scope of the claims of the present invention, is that the catalyst structure appears to affect the type and number of deviations in the chain from the principally repeating units in the polymer. Previously, the catalyst used to produce syndiotactic polypropylene were believed to exercise chain-end control over the polymerization mechanism. These previously known catalysts, such as the ones disclosed by Natta et al in the references cited above, produce syndiotactic polymers which primarily have the following structure: ##STR3## or in NMR nomenclature . . . rrrmrrr . . . The NMR analysis for this structure of syndiotactic polypropylene is shown in Zambelli, et al., Macromolecules, Vol. 13, pp 267-270 (1980). Zambelli's analysis shows the predominance of the single meso dyad over any other deviations in the chain.